In an exhaust gas purification apparatus in which urea is supplied to an NOx selective reduction catalyst (hereinafter also referred to simply as an “NOx catalyst”), there has been known a technique in which when an amount of intermediate products, which have been generated in the course of the reaction from urea to ammonia and which have been accumulated in an exhaust passage, reaches an upper limit amount, the supply of urea water is prohibited (see, for example, a first patent document). According to this technique, a reducing agent can be supplied to the NOx catalyst until the amount of the intermediate products accumulated in the exhaust passage reaches the upper limit amount.
However, in the exhaust passage, there may be provided with a filter for trapping particulate matter (hereinafter also referred to as “PM”). In addition, in order to determine a failure of this filter, there may be provided with a PM sensor which serves to detect an amount of particulate matter in an exhaust gas. When the above-mentioned intermediate products adhere to an electrode or a cover of this PM sensor, there is a fear that it may become difficult to detect the amount of particulate matter in an accurate manner. In that case, there is also a fear that the accuracy in failure determination of the filter may become low.